September 2013

Largest radio telescopes in the world are used by professional radio astronomers, and often you can also visit them. Radio telescopes are extraordinary instruments, equipped with giant parabolic antennas or other, designed to work as single instruments or as interferometers. They are used to study objects in the Universe in radio waves frequencies but often are also used for satellite communication or studies of Earth’s atmosphere. Here you have a list with some of the largest radio telescopes in the world and a brief description for each instrument.

Very Large Array – VLA (USA)
Probably one of the most famous radio telescopes in the world thanks to films like “Contact”, it uses 27 Cassegrain antennas each 25 meters diameter that can be moved along a Y shaped rail system.

Largest radio telescopes: VLA (Credit: Alex Savello)

Arecibo (Puerto Rico)
Up to 2016, it was the largest parabolic antenna in the world, thanks to its 305 meters diameter. The antenna was placed on a natural depression in the ground and it has no mount: the radio telescope can point different sky regions moving the central feedhorn.

Largest radio telescopes: Arecibo (Credit: Arecibo Observatory)

GBT (USA)
The Robert C. Byrd Green Bank radio telescope has a parabolic antenna with asymmetric surface and an off-axis illumination. In Green Bank there are also other large radio telescopes such as the 43-meter diameter one with equatorial mount.

Largest radio telescopes: GBT (Credit: NRAO/AUI/NSF)

Atacama Large Millimeter/submillimeter Array – ALMA (Chile)
The ALMA radio telescope includes many 7 and 12 meters diameter parabolic antennas that have been installed in Atacama desert in Chile, about 5000 meters above sea level. Thus, it will study also the high radio frequencies usually blocked by the atmosphere.

Largest radio telescopes: ALMA (Credit: NRAO/AUI/NSF)

FAST (China)
The Five-hundred-meter Aperture Spherical radio Telescope (FAST)) is a radio telescope located in southwest China. It consists of a fixed 500 m diameter dish constructed in a natural depression in the landscape and it is the world’s largest filled-aperture radio telescope.

Largest radio telescopes: FAST (Credit LIU XU)

Effelsberg (Germany)
Thanks to the huge 100 meters diameter parabolic antenna, this is one of the largest radio telescopes in the world. This radio telescope weighs 3200 tons and it takes 12 minutes to make a complete 360 degrees rotation.

Largest radio telescopes: Effelsberg (Photo by CEphoto, Uwe Aranas)

Medicina (Italy)
Near Bologna there are two radio telescopes: the “Northern Cross” that consists of an array of antennas in two perpendicular arms and a 32 meters diameter parabolic antenna which is also used in interferometric observations.

Largest radio telescopes: Medicina (Credits: Filippo Bradaschia)

Sardinia Radio Telescope (Italy)
This radio telescope, built 35 kilometers away from Cagliari, uses a 64 meters diameter parabolic antenna designed with high accuracy (among the best of several radio telescopes in the world) in order to allow recording at high frequencies (up to 100 GHz).

Largest radio telescopes: SRT (Credits: INAF)

Lovell Radio telescope (England)
With its 76 meters diameter antenna, this instrument is one of the largest radio telescopes in the world with movable reflector. It is located in Jodrell Bank (England) and it’s part of English MERLIN interferometer system.

Largest radio telescopes: Lovell (Credits: Mike Peel; Jodrell Bank Centre for Astrophysics, University of Manchester)

Parkes (Australia)
Parkes Observatory is located in the south-eastern Australia and it uses a great 64 meters diameter parabolic antenna. In addition to radio astronomy, it was also used to collect the Apollo 11 transmissions coming from the Moon.

Largest radio telescopes: Parkes (Credits: Stephen West)

Square Kilometer Array – SKA
Currently under study, it uses a network of thousands of antennas installed both in Australia and in South Africa. Combining the recorded signals, it will be possible to obtain a collecting area equivalent to the one of 1 square kilometer parabolic antenna.

What are the results of radio telescopes? In this article we’ll see some like radio images, transits and numbers. When we record a picture of an Universe object, we usually use a digital camera that has many pixels (typically several millions). This way, when we record the picture, the light we receive “lighten” many pixel at the same time and each pixel records light coming from different sky areas.

But, when we use radio telescopes, we record the signal from a single area of the sky (except for professional radio telescopes that may have more LNA units), just as if our camera had only one pixel. If the instrument is equipped with a precise automatic pointing and tracking system, and you have the coordinates of many sky radio sources of the Universe (as in the case of our SPIDER radio telescope), you can point the antenna to the correct direction, and then record the radiation flow coming from the object itself.

This expresses the power of the signal emitted by the radio source for frequency unit that passes through a surface of unit area. The type of data obtained depends on the characteristics of the receiver and by the fact that the measure could be calibrated or not. That’s why, in general, the first result you get radio telescopes pointing towards the sky is a number.

Results of radio telescope: on the left, radio telescopes record the radio waves coming from a specific area of ​​the sky. On the right, radio telescopes can also record transits of the radio-source to study.

Another result type that can be obtained with radio telescopes is a transit. This technique is to identify the object for which you want to record the radio emission, point the telescope in the sky area in which the object will move in the near future (eg 30 minutes later) and stop the telescope in that position. Because of the apparent sky rotation (caused by the rotation of Earth), the object will move towards the area of sky pointed by the antenna (a), there will be (b) and will pass (c). Another way to do this is to move the radio telescope from point a to c with a faster speed than sidereal rate.

We can thus record a curve of values ​​whose central bell shows the recording of the radiation flux emitted by the radio source and that is captured by the antenna main lobe. On left and right of the graph, you can see two lower increases in signal due to the secondary lobes of the antenna. This type of result is very interesting since it allows to evaluate other parameters such as the resolution capabilities of the antenna and is also used to verify the performance and working settings (such as focus point).

Results of radio telescopes: The transit of the Sun recorded by SPIDER radio telescope. Observe the signal increases due to the passage of the Sun in the main lobe and side lobes. The X axis corresponds to time while the Y (vertical) one corresponds to the intensity of the signal, represented in logarithmic scale to better highlight the secondary lobes.

If the radio telescope has a precise tracking and automatic pointing system, we can record a radio image of the object that we want to study. But how can we achieve this if our radio telescope only records one pixel at a time? In order to do so, the radio telescope is moved continuously by scanning the sky area desired and recording, from time to time, the radio emission which comes from each pixel that compose the image. Some pixels will record a different quantity of radio waves from the adjacent ones and this amount is recorded by radio telescope. Therefore, each number is associated with a color: the computer will substitute numbers with colors chosen by generating a radio image of the object again!

Our SPIDER radio telescopes allow the user, in a very simple way, to obtain these results with an amateur radio telescope. Whatever you want to accomplish, you can register waves easily using RadioUniversePRO software. In the image below, you can see a radio map of the Sun shown here each pixel has a size of 1.5 degrees. There are also the effects of side lobes (the blue “spots” around the Sun).

Results of radio telescopes: radio-Image of the Sun recorded with SPIDER radio telescope. Each pixel corresponds to a numerical value proportional to the intensity of the radio emission coming from a precise sky area.